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Review

Lunar Mare Fecunditatis: A Science-Rich Region and a Concept Mission for Long-Distance Exploration

by
Siyuan Zhao
1,
Yuqi Qian
1,
Long Xiao
1,*,
Jiannan Zhao
1,
Qi He
1,
Jun Huang
1,
Jiang Wang
1,
Hui Chen
2 and
Weiyang Xu
2
1
State Key Laboratory of Geological Processes and Mineral Resources, Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2
Aerospace System Engineering Shanghai, Shanghai 201109, China
*
Author to whom correspondence should be addressed.
Remote Sens. 2022, 14(5), 1062; https://doi.org/10.3390/rs14051062
Submission received: 8 January 2022 / Revised: 17 February 2022 / Accepted: 18 February 2022 / Published: 22 February 2022
(This article belongs to the Special Issue Planetary Geologic Mapping and Remote Sensing)
Browse Figures
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Abstract

:
Mare Fecunditatis is a ~310,000 km2 flat basalt plain located in the low-latitude area of the Moon. Plenty of volcanic features (multiple episodes of mare basalts, sinuous rilles, lava tubes, pyroclastic deposits, domes, irregular mare patches (IMP), ring-moat dome structures (RMDS), floor-fractured craters), tectonic features (grabens and wrinkle ridges), impact-related features, and other features (swirls, pit craters) are identified in Mare Fecunditatis. An in-situ mission to Mare Fecunditatis is scientifically significant to better understand the lunar thermal histories and other questions. All previous in-situ and human missions (Apollo, Luna, Chang’E) were limited to small areas, and no traverse longer than 40 km has been made yet. With the development of technology, long-distance movement will be possible in the future on the lunar surface, providing opportunities to explore multiple sites at one mission with complete documentation of the regional geology. Eight high-value targets (pit crater, IMPs, RMDSs, young basalts, high-Al basalts, pyroclastic deposits, swirls, and fresh craters) were found in Mare Fecunditatis, and a ~1400 km-traverse in 5 years is proposed to explore them to solve the most fundamental lunar questions.

Graphical Abstract

1. Introduction

The Soviet Union, United States, Europe, China, Japan, and India have successively conducted their lunar robotic or crewed missions since the 1960s. Extraordinary scientific discoveries have been made, but even more questions have been proposed, including (1) bombardment history, (2) lunar interior, (3) crust and regolith, (4) polar region and volatiles, (5) volcanism and thermal evolution, (6) impact process, and (7) space weathering [1], and more scientific landing sites across the Moon have been proposed (e.g., [2,3,4,5,6,7]). However, different high-value targets are distributed all over the Moon and are normally far away from each other (Figure 1). The short detection distance (less than 39 km, made by Lunokhod 2 in 1973) of previous missions limited the complete scientific documentation of the investigation area, and exploring multiple sites in large distance at one mission was impossible in the era. With the development of technologies, multi-target long-distance missions [8,9] in the near future may become reality. The Intrepid mission [8] concept is one of the examples. The designed rover plans to travel ~1800 km in the Oceanus Procellarum in a 4-year period, which will explore Reiner Gamma, Marius Hills, young mare basalts, Aristarchus Plateau, Aristarchus Crater, and irregular mare patches (IMPs). Both 4 and 1.2 billion years of volcanic activity on the Moon could be revealed by Intrepid. The long travel distance would make the scientific outcomes several times larger than a normal rover mission.
Mare Fecunditatis (centered at 7.8° S, 53.7° E) is a low-latitude area with flat terrain on the nearside of the Moon. The basin formed in the pre-Nectarian period (>3.92 Ga), while the mare basalts were mainly filled in the Imbrian period (3.2–3.85 Ga) and continued until the Eratosthenian period (1.1–3.2 Ga) [10,11,12]. There is a diversity of volcanic features (e.g., mare deposits, sinuous rilles, lava tubes, pyroclastic deposits, domes, irregular mare patches, ring-moat dome structures), tectonic features (e.g., graben and wrinkled ridge), impact-related features, and other features (e.g., swirls, pit craters) in Mare Fecunditatis. A composite of in-situ investigations of these features will provide a window to understand the external and internal processes. In this study, the scientific value of eight targets (pit crater, IMPs, RMDSs, young basalts, high-Al basalts, pyroclastic deposits, swirls, and fresh craters) was listed, and a ~1400 km traverse routine through Mare Fecunditatis in 5 years was proposed, which would help the planning of long-distance missions in the future.
Remotesensing 14 01062 g001 550
Figure 1. Orthographic map of Wide Angle Camera (WAC, [13]) image with projection center in Mare Fecunditatis (7.8° S, 53.7° E). The flag icons stand for the previous soft-landing missions of the United States, Soviet Union, and China, respectively. The green points, dark yellow triangles, blue points, red triangles, pink outlines, green outlines, and jadeite areas represent identified pit craters [14], IMPs [15], RMDSs [16], pyroclastic deposits [17,18], swirls [19], fresh craters [20], and young basalts (≤3.0 Ga, [21,22,23,24,25]), respectively. The white box represents the locations of other figures in this paper.
Figure 1. Orthographic map of Wide Angle Camera (WAC, [13]) image with projection center in Mare Fecunditatis (7.8° S, 53.7° E). The flag icons stand for the previous soft-landing missions of the United States, Soviet Union, and China, respectively. The green points, dark yellow triangles, blue points, red triangles, pink outlines, green outlines, and jadeite areas represent identified pit craters [14], IMPs [15], RMDSs [16], pyroclastic deposits [17,18], swirls [19], fresh craters [20], and young basalts (≤3.0 Ga, [21,22,23,24,25]), respectively. The white box represents the locations of other figures in this paper.
Remotesensing 14 01062 g001

2. Topography of Mare Fecunditatis

Mare Fecunditatis is located in the low-latitude area on the eastern nearside of the Moon, covering an area of ~310,000 km2 (Figure 1 and Figure 2a). SELENE and LRO DEM 2015 (SLDEM2015, 60 m/pixel, [26]) data show that Mare Fecunditatis has a higher elevation in the southwest and a lower elevation in the northeast, and the elevation of 99% of Mare Fecunditatis varies from −0.3 km to −2.7 km (Figure 2b). The peak ring of the Mare Fecunditatis basin is strongly degraded, and the outline is difficult to distinguish [27]. The slope in the basin interior is relatively gentle (average slope of 3.56°); more than 99% of the area has a slope of less than 20°, and 84% of the area has a slope of less than 5° (Figure 2c). Roughness [28] based on the median slope of the 57 m-baseline Lunar Orbiter Laser Altimeter (LOLA, [29,30]) data shows an average value is 1.7°, while the roughness of more than 90% of the area is less than 2.6° (Figure 2d). Mare Fecunditatis is relatively flat, and only crater rims appear with large roughness.

3. Geology and Geomorphology of Mare Fecunditatis

Mare Fecunditatis basin was formed in the pre-Nectarian period, followed by the mare basalts eruption in the Imbrian period, and the volcanic activity continued until the early Eratosthenian period [10,11,12]. There is no mass concentration in the center of Mare Fecunditatis [31], while there are positive Bouguer anomalies on the east and west sides [32] of the basin. A diversity of geological features (Figure 3) is found in Mare Fecunditatis.

3.1. Volcanic Features

3.1.1. Mare Basalts

Lunar mare basalt is the main volcanic product of lunar mare volcanism, originated from the partial melting of the lunar mantle and filling some of the impact basins after eruption, including Fecunditatis basin, forming Mare Fecunditatis.
The basalt in Mare Fecunditatis has an average thickness of ~500 m [36], with a thickness of up to 1500 m in the center [37]. The Multiband Imager (MI, [38]) FeO map shows that Mare Fecunditatis is 17–20 wt% in the center and 14–17 wt% in the south (Figure 2a, [39,40]). The WAC TiO2 map [41] shows high-Ti (TiO2 > 6 wt%) and low-Ti (TiO2: 2–6 wt%) basalts are mainly distributed in the center of the basin, while very low-Ti (TiO2 < 2 wt%) basalts are mainly distributed in the and southern region (Figure 2b). Interference Imaging Spectrometer (IIM, [42,43]) Al maps [44] show high-Al (Al2O3 > 11 wt%) basalts are widely distributed in the Mare Fecunditatis, except for the northeastern part; the results are similar with the compositional constraints based on FeO, TiO2, and Th maps [45,46].
Luna 16 landed in the northeastern part of Mare Fecunditatis (0.7° S, 56.3° E) in 1970. All returned basalt fragments have high-Al (Al2O3 > 11 wt%) characteristics [47], with a TiO2 content of 1–5 wt%, Al2O3 content of 11–20 wt%, and Mg# (Mg/(Mg+Fe) in mole percent) of 0.3–0.4 [47,48]. The isotopic ages of basalt samples show three-episode magmatic activities between ~3.14 and 3.42 Ga, but concentrate in ~3.4 Ga [49,50,51,52]. According to the mare unit map [12] (Figure 4d), the age of the unit where Luna 16 landed is 3.36 Ga, which is in good agreement with the 3.4 Ga isotopic age. The age of the low-Ti, high-Al basalt in the central and southern parts is 3.5–3.7 Ga, while the high-Ti basalt in the northeast is relatively young, at 3.34–3.36 Ga, and the magmatic activity of some basalts in the southwest continues to 3.14 Ga. In addition, we find a small number of areas with lower albedo, fewer craters, and fewer ejecta materials. Among them, three areas show relatively younger ages (2.2–2.9 Ga; Figure 5).

3.1.2. Sinuous Rilles

Sinuous rilles are formed by the erosion of the lunar surface or the continuous collapse of lava tubes during the flow of high-temperature lava, which usually originates from a depression, extending from high terrain to low terrain, and gradually disappears on the smooth lunar mare [54]. A short small-scale sinuous rille (Figure 3 and Figure 6) is found in the western part of Mare Fecunditatis, adjacent to pyroclastic deposits. This sinuous rille is 26.8 km in length, 1000 m wide, and 69 m deep. A possible 17.5 km long collapsed lava tube with 0.69 km in width and 65 m in depth is also discovered next to the sinuous rille.

3.1.3. Floor-Fractured Craters

The Taruntius crater (5.5° N, 46.5° E, Figure 7a) and Goclenius crater (10.0° S, 45.0° E, Figure 7b) are found in the northern and western part of the Mare Fecunditatis, respectively. Their floors are cut by concentric or polygonal fractures, known as floor-fractured craters (FFCs) [35,55,56]. There are two formation hypotheses for FFCs: (1) viscous relaxation, wherein the crater floor rebounds to fill the crater at a rate controlled by the subsurface viscosity structure, resulting in an overall amplitude shallowing of long-wavelength crater topography [57]; and (2) magma intrusions and sills form beneath the crater, and the magma lifting produces the laccolith and fractures the overlying the crater floor [35,55,56,58]. Recent numerical simulations do not support the viscous relaxation hypothesis [59], but high-resolution topographic and gravity data advocate the magma intrusion hypothesis [35,55,56].

3.1.4. Pyroclastic Deposits

Pyroclastic deposits are the products of explosive volcanism, and the characteristic richness in glass and titanium make them darker than overflow basalts [60]. They have a smooth surface and usually occur close to sinuous rilles, irregular depressions, and the boundary between the maria and highlands [61]. Two areas in Mare Fecunditatis have pyroclastic deposits: in the Taruntius crater [62] on the northern side of the center peak (Figure 7a, 5.4° N, 46.5° E), and the area connecting to the sinuous rille (Figure 6, 3.0° S, 42.3° E).

3.1.5. Irregular Mare Patches

Irregular Mare Patches (IMPs) are enigmatic features occurring in the lunar mare. Typical IMPs (such as Ina (18.65° N, 5.30° E) and Sosigenes (8.335° N, 19.071° E), Figure 1) are one to several kilometers in size and composed of positive-relief mounds surrounding low rough hummocky and/or blocky floor units. The IMPs in Mare Fecunditatis are relatively small (tens to hundreds of meters in size) and only develop irregular, rough, bright, and pit features. They may be formed by (1) sublimation [63]; (2) small magma intrusion on the top of the dome [64,65,66]; (3) clearing of the overlying lunar regolith by outgassing activity within 10 Ma [67]; (4) lava flow inflation [68]; (5) basalt eruption within 100 Ma [15]; (6) pyroclastic eruption [69]; and (7) lava lake process and foamy magma extrusion [70,71,72,73]. The most fundamental scientific question of IMP is whether they are young or not. Some authors [15] suggest that IMPs have ages younger than 100 Ma based on impact crater chronologies. Others proposed IMP formed contemporarily with the Imbrian-aged host basalts [68,70].
In the western part of Mare Fecunditatis, dozens of IMPs (Figure 3) are identified within a length of tens to hundreds of meters. They are distributed on three small Eratosthenian-aged areas (Figure 5), where TiO2 content is >4 wt%. Most of the IMPs are located next to the rim of craters (Figure 8a), and a few IMPs are not connected to the crater (Figure 8b).

3.1.6. Domes

The lunar volcanic domes are formed by (1) cooling limited lava flows [74]; (2) subsurface intrusion [75]; and combination of (2) and (1) [76]. There are at least 38 domes (e.g., Figure 9) with a diameter of more than 500 m in the Mare Fecunditatis, mainly distributed in the center of Fecunditatis where the content of TiO2 generally exceeds 3 wt%.

3.1.7. Ring-Moat Dome Structures

RMDS is a newly discovered lunar volcanic feature [77]. RMDSs are small circular mounds hundreds of meters in diameter and ∼3–4 m in height, surrounded by narrow, shallow moats (Figure 10). Four hypotheses were proposed for the formation of RMDSs [77]: (1) high-viscous lava eruption soon after the mare basalt; (2) geologically very recent eruption; (3) small-scale squeezing features formed at the time of the mare basalt; and (4) volatility-rich magmatic foam extrusion. More than 1,600 RMDSs have been identified in the Mare Fecunditatis [16] and they are concentrated in the northern sector, where TiO2 contents of the RMDS-bearing basalt are larger than 3 wt% and absolute model ages range from 3.36 to 3.67 Ga [12]. The height of RMDSs is very low (usually a few meters), which should be flattened now if RMDSs have similar ages with the surrounding mare basalts. Some RMDSs were found in the craters with a low degree of degradation (130–1500 Ma); thus, RMDSs are also potentially very young features [78,79].

3.2. Tectonic Features

3.2.1. Wrinkle Ridges

A wrinkle ridge is a linear positive relief landform on the lunar surface, which extends up to hundreds of kilometers, mainly distributed in the lunar maria. There are three main hypotheses on the formation of wrinkle ridges: (1) tectonic origins [80,81,82]; (2) magmatic origins [83,84,85]; and (3) tectonic and magmatic origins [85,86,87,88]. There are more than two hundred wrinkled ridges (Figure 3) in Mare Fecunditatis [89]. Their lengths vary from 1 km to 250 km; most of them are less than 50 km long, mainly with an NS trend, consistent with the global trend [34].

3.2.2. Arcuate Rilles and Grabens

In addition to the sinuous rilles and floor fractures, the tectonic movement also forms long and narrow grooves, including arcuate rilles and grabens (straight rilles) (Figure 6). Arcuate rilles are usually on the edge of the lunar mare with smooth curves [90]. The graben (straight rilles) forms under the extensional stresses, and a block of the crust cracks and drops down to create the valley floor [91,92,93].

3.3. Impact Craters

Impact craters are the most common geomorphologic features on the lunar surface. According to their size and shape, craters are divided into simple craters (<20 km, bowl-shaped, Figure 11a), complex craters (tens to hundreds of kilometers, with terraced walls, central peaks, and flat floor), and multi-ringed basins (>290 km, with multiple peak rings) [94]. Impact craters in Mare Fecunditatis are mainly simple impact craters, with several complex impact craters on the edges. There are also some atypical craters such as elliptical craters (Figure 11b), buried craters (Figure 11c), and crater chains (Figure 11d) in Mare Fecunditatis.
Messier Crater (1.9° S, 47.67° E) is an elliptical crater with a major axis ~14.3 km and a minor axis of ~8.3 km, while the Messier A crater (1.97° S, 46.95° E) has a major axis ~15.8 km and a minor axis ~11 km (Figure 11b) [95]. Messier was likely formed by a low-angle westward impact, and Messier A formed following a rebound by the impacting body [96]. Major vertical rays extend over 100 km north and south from Messier and horizontal rays extend over 100 km west from Messier A. The results of Monte Carlo simulations [97] show that the simulating ray patterns by oblique impact are very similar to reality. In addition, there are at least 29 buried impact craters in the Mare Fecunditatis. The crater chains refer to a row of craters distributed linearly. Crater chains formed by secondary craters and collapse of the lava tube are identified in Mare Fecunditatis.

3.4. Other Features

3.4.1. Pit Craters

Lunar pit craters formed by the collapse of underground space. A total of 15 pit craters have been discovered on the maria and 5 on the highlands [14,98,99,100]. The underground space may be formed by the lava tube or the stoping of magma [14,100]. Recently, the underground space of the caves in Marius Hills has been detected through radar and gravity analyses [101,102]. Pit craters stay stable for at least tens of years. Some pits (such as King Crater Bridge) found at the Apollo period still show no change with the NAC data [14]. There is a pit crater (0.917° S, 48.660° E, Figure 12) in the center of Fecunditatis, and a highland-type pit crater (6.752° S, 42.759° E, Figure 13) at the southwest.
This paper employs Integrated Software for Imagers and Spectrometers (ISIS) to make the high-resolution NAC DTM (Figure 12c and Figure 13c) of two pit craters. The entrance to the central Mare Fecunditatis pit crater (Figure 12) is about 125 × 100 m in size and 35 m in depth. Deposits lie on the southeast and northwest wall. The deposits on the northwest wall extend ~30 m laterally, with a slope of 20–65°. The southeast wall has collapsed severely, with a relatively gentle slope (10–35°). Most of the deposits are finer than the resolution of the NAC image (~1.1 m), with only a few meter-level rocks at the bottom of the pit crater. The bottom of the pit is relatively flat (<10°), with an area of 27 × 23 m2 lower than 15°. The highland pit crater on the southwest side of Mare Fecunditatis (Figure 13) is about 70 m in diameter, and the walls are very steep in all directions. The gentlest southwest wall is 30–60°, and the maximum slope of the northeast wall exceeds 80°. The bottom of the pit is relatively flat (<10°) with an area of 23 × 10 m2.

3.4.2. Swirls

Lunar swirls are high-albedo loops and ribbons occurring in both maria and highlands associated with strong crustal magnetic fields. The formation of the swirls may be related to (1) the magnetic anomalies blocking the solar wind ion bombardment, which reduces the degree of space weathering (darkening with time) [103,104,105,106]; (2) comet impacts or micrometeoroid swarms scouring the top-most surface regolith, which exposes fresh material and imparts a remnant magnetization [107,108,109]; (3) weak electric fields attract the high-albedo, fine-grained, feldspar-rich dust [110]. The interaction between the solar wind and silicate regolith produces water (e.g., [111,112]). Moon Mineralogy Mapper (M3, [113]) spectral data show that the regolith on the swirl has lower water than the surrounding area [106], which supports the hypothesis (1).
There are three swirls (Figure 14, identified by [19]) on the highland at the southwest side of Mare Fecunditatis with a low optical maturity (OMAT, Figure 14b, [40]), and the surface vector mapping (SVM, [114]) of magnetic field (B-flied) shows this area has a magnitude of up to 100 nT (Figure 14d). The MI mineral data (Figure 14c, [40]) of the swirl show consistent plagioclase with the surrounding area, which contradicts hypothesis (3).

4. Discussion

After analyzing the geological features in Mare Fecunditatis, eight targets are selected with high scientific values. First, we summarize their scientific significance and then design a traverse to detect all of these targets for a composite long-distance mission.

4.1. High-Value Detection Targets

4.1.1. Pit Craters

Pit craters are the natural cross-section to the subsurface of mare basalts, which could be used to study multiple eruptions of lunar volcanism and ancient regolith. Pit craters also connect to underground space, which has excellent conditions for the construction of a long-term lunar research station [115,116], such as (1) relatively constant temperature [117,118], (2) low cosmic ray flux [119,120], (3) little lunar dust [98], and (4) shielding from micrometeorite impact [120]. However, the entrance of the pit crater is usually very steep. The pit crater in the middle of Mare Fecunditatis has a collapsed wall (10–35°) comprised of deposits on the southeastern side, which makes it easy to enter the underground space technically. A robot with inflatable composite structures [121] with laser scanners [122] and other payloads may be considered to enter the cave. If successful, this project be the first to enter the moon and investigate in-situ lava outcrops.

4.1.2. Young Volcanism

Crater size–frequency distribution (CSFD) measurements are widely used to date the planetary surface. The CSFD measurements rely heavily on the lunar crater chronology function (e.g., [123,124,125,126,127]), which is based on the returned lunar samples with known sources. However, due to the lack of young samples (<3 Ga) at the age range of 1–3 Ga, the chronology function maybe not be constrained well [128]. The lunar chronology function is gentle, between 1 and 3 Ga, plus there is a small crater population in the young region, which will bring additional errors of dating a young lunar surface [129]. Although Chang’E-5 brought back some young lunar basalts, with the youngest dating age being ~2.0 Ga [130,131], more samples are needed to better constrain the lunar chronology function within the age between 1.0 and 3.0 Ga. Basalts in Mare Fecunditatis with ages between ~2.2 and ~2.9 Ga are ideal places to constrain the chronology function.
The timing of the latest lunar volcanic activity is extremely important for understanding the thermal evolution history of the Moon. The Moon is expected to cool rapidly and freeze the molten mantle layer quickly after its formation due to its small size [132]. Some authors simulated that the heat is enough to maintain partial melting from the mantle and eruption of mare basalts, until 2.2–3.4 Ga [133]. The youngest mare basalts are mainly distributed in Oceanus Procellarum (e.g., [21,25,134]) with the concentration of heat-producing elements (e.g., [135,136,137]). However, the 2.0 Ga Chang’E-5 basalts are neither admixing KREEP (enriched in potassium, rare-earth elements, and phosphorus) [130,138,139] nor enriching volatiles [140], raising the question of what derives the late-stage basalts. The young mare basalts within Mare Fecunditatis are far away from the Lunar Procellarum KREEP Terrane (PKT) and may have a distinct origin with Chang’E-5 samples. All of the IMPs are emplaced in the Eratosthenian basalts, and RMDSs are distributed in the Imbrian basalts with TiO2 > 3 wt% in Mare Fecunditatis; their spatial and time relationship may provide additional clues of the young lunar volcanism. Plenty of in-situ chemical detections, sample returns, and rock dating are required, and it is noteworthy that there may be some sub-meter rocks in IMPs.

4.1.3. High-Al Basalts

Lunar basalt samples have significantly enhanced our understanding of the lunar mantle composition and structures. High-Al basalts are widely distributed across the Moon [45,46]. However, high-Al basalt samples are relatively few (Apollo 12, Luna 16, and one sample each from Apollo 14 and Apollo 16). Moreover, the age of these samples is bimodal, with few 3.4–4.0 Ga-old samples and no 3.5–3.8 Ga-old samples. Remote sensing data show that the southern Mare Fecunditatis is filled with low-Ti (<2 wt%) high-Al basalts of 3.6–3.7 Ga [60], which is distinct from the collected samples. Thus, the high-Al basalts in the southern Mare Fecunditatis could provide samples to study this type of special lunar volcanism.

4.1.4. Pyroclastic Deposits

Pyroclastic deposits result from explosive volcanic eruptions, originating from the deep mantle (~300 to ~400 km) [141,142]. The pyroclastic magma rises faster and may carry some lunar mantle xenoliths. Sample analysis (e.g., [143,144,145,146]) and infrared remote sensing data [147,148] showed over a hundred μg/g water in the pyroclastic glasses, indicating the existence of volatile reservoirs in the lunar mantle. More pyroclastic glasses are required to understand the water content and distribution in the lunar mantle. On the other hand, the water in the pyroclastic deposits may be an important in-situ resource.
There are two sites of pyroclastic deposits in the Mare Fecunditatis: (1) one in Crater Taruntius (5.4° N, 46.5° E), whose wall has a relatively high slope (15–30°), but is difficult to access; (2) another in the western region of the Mare Fecunditatis (3.0° S, 42.3° E), which is flat and easy to access.

4.1.5. Swirls

Lunar swirls are essential to understand (1) the history of the lunar core dynamo; (2) the magnetic effects of large-scale impacts; (3) the formation of the swirls; and (4) the specific space weathering effects (including the production of nanophase iron and water) to the regolith with weak solar wind ion bombardment. Hundreds of swirls have been identified on the lunar mare and highlands. However, none of the swirls have been detected by the in-situ missions. Lunar swirls on the west side of Mare Fecunditatis are across both highland and mare and most areas have a slope less than 15°, which is possible to detect. Swirls near Mare Fecunditatis are across the highlands and maria, which require detailed route planning with high-resolution images (e.g., NAC) and a rover with excellent climbing ability.

4.1.6. Fresh Craters

The nature and extent of lateral and vertical mixing of local and ejecta material are poorly constrained [1]. The eject rays from the fresh craters are less affected by space weathering, and it is easy to separate the young ejecta from local materials if they have distinct compositions. Petavius B (19.9°S, 57.0°E, 32.0 km in diameter), located in the southeastern Mare Fecunditatis, is the ideal target to study the mixing of local and ejected materials. It has a high albedo butterfly-shaped ray pattern stretching ~200 km with low-Fe and low-Ti contents.
In addition, sample returned from impact melts of the fresh crater, such as impact melts (tens of km2) between Messier and Messier A, would provide additional calibration points at the younger range of the lunar chronology function, as a supplementary of the young impact melt samples of Copernicus, Tycho, North Ray, and Cone craters collected in the Apollo missions.

4.2. Traverse Design for Long-Distance Detection

Here, we envision a long-range exploration mission, referring to previous rovers and the Intrepid [8,149] (Table 1). The concept mission temporarily named Fengfu is under the brainstorming stage, and some content may be unrealistic right now. Most of the high-value detection targets selected in this study are distributed in the center and southern Mare Fecunditatis. A 1408 km traverse (Figure 15; Table 2) is designed with reference to the 60 m spatial resolution SLDEM2015 slope map and 10 m-spatial-resolution TC images [150]. The detection traverse starts from an RMDS (S1-1, 1.31° S, 52.50° E), passing through several RMDSs (S1), domes (S1-4 and S3), pit crater (S2), Messier Crater and its impact melt and ray system (S4), young mare basalt (S5), IMPs (S5), pyroclastic deposits (S6), sinuous rilles (S6), high-Al basalt, swirls (S7), and impact blanket of the Petavius B. The average slope of the traverse is 2.58°, the slope of more than 94% in the traverse is less than 5°, and almost 100% has a slope of less than 10°. After entering the swirl on the highland, the slope is steeper, but still <20°.
The traverse needs an unmanned rover with scientific payloads and some small robots for sample collections. A small rover is greatly affected by the ambient temperature; for example, Yutu-2 (135 kg) has to finish working at noon due to the high temperature. Although a large rover is likely to have a good temperature-control system, we assume that the Fengfu rover only has a third of the time (236 h) to work per lunar day and has two working modes: (1) target-exploration mode, focusing on exploring targets and hardly moving; and (2) navigation mode, traveling and detecting passing regolith and rocks. Fifteen lunar days are allotted to the target–exploration mode (Table 2), including five lunar days for the pit crater, and one lunar day for another target. Complex targets (such as pit craters) would be explored in conjunction with astronauts or dedicated robots at the same time—the samples would be returned, and the rover would be repaired. Although the Fengfu could be designed to move very quickly, the rapid movement of the lunar rover faces the risk of dust accumulation [151,152]. If the average navigation speed of the Fengfu is 0.5 km/h (the same as the Intrepid), and (semi-automatically) travels 30 km (60 h) every lunar day, there are another 176 h to explore passing regolith and rocks (roughly traveling for 1 h and exploring for 3 h). It will take ~47 lunar days to finish the navigation. At the end of this mission, the rest of samples would be packeted and returned by another mission. A total of 62 lunar days (~5 years) are required to complete the mission.
Compared with detection targets in the Intrepid mission [8,149], both missions contain IMPs (potential very young volcanism), young basalts (modification the crater chronology curve and understanding the lunar thermal history), pyroclastic deposits (understanding the deep mantle), and swirls (understanding lunar core dynamo and space weathering effects). The targets in our work also include the pit crater (to detect underground space), RMDSs (potential very young volcanism), high-Al basalt (filling the gap between Apollo 14 and Luna 16), and large fresh craters with melt sheet (modification of the crater chronology curve) or distinct composition (understanding the mixture of local and ejecta material processes). The exploration of Mare Fecunditatis is significant to fully understand the history of lunar evolution.

5. Conclusions

A long-distance lunar traverse could solve more fundamental scientific questions compared with a nominal short-distance mission. Mare Fecunditatis has a multitude of high-value features and a flat terrain, which is very suitable for long-distance detection. In total, several outstanding detection targets are selected: (1) pit crater may connect vast underground void space, which is an ideal place to build a research station; (2) young basalts (2.2–2.9 Ga) and the impact melts of Messier Crater may modify the chronology functions; (3) young basalts and potential very young volcanism (IMP and RMDS) are key to the thermal evolution history of the Moon; (4) basalts in the southern Fecunditatis may be a type of high-Al basalt that differs from previous samples; (5) pyroclastic deposits are windows to understanding the composition of the deep mantle; (6) swirls and magnetic anomalies are important to the history of lunar core dynamo and space weathering effects; and (7)the ray system of the fresh crater may disclose the mixing of local and ejecta material processes. Based on this, a ~1408 km traverse over 5 years has been proposed to explore Mare Fecunditatis, which may help future planning of this class of long-distance missions on the lunar surface.

Author Contributions

Conceptualization, L.X.; methodology, S.Z.; investigation, S.Z.; writing—original draft preparation, S.Z.; writing—review and editing, S.Z., Y.Q., L.X., J.Z., Q.H., J.H., J.W., H.C. and W.X.; visualization, S.Z.; funding acquisition, L.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R & D Program of China (2020YFE0202100) MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (MSFGPMR05), and the 111 project of China (BP0820004).

Data Availability Statement

The WAC images, NAC images, SLDEM2015, Clementine color ratio map, WAC TiO2 map, and MI mineral and composition maps are available at PDS Geosciences Node (https://pds-geosciences.wustl.edu/, accessed on 7 January 2022). The TC images and TCDTM are available at SELENE Data Archive (https://darts.isas.jaxa.jp/planet/pdap/selene/index.html.en, accessed on 7 January 2022). The IIM chemical maps are available at https://mega.nz/#F!6ThgVK6a!_x1Pq1p5UW4SVvbK640wow, accessed on 7 January 2022. The SVM magnetic field maps are available at http://www.geo.titech.ac.jp/lab/tsunakawa/Kaguya_LMAG, accessed on 7 January 2022.

Acknowledgments

The authors would like to thank three anonymous reviewers for their insightful and constructive comments and suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. The (a) WAC image. The white boxes represent the locations of other figures in this paper (b) SLDEM2015, (c) SLDEM2015 slope, and (d) LOLA roughness of Mare Fecunditatis.
Figure 2. The (a) WAC image. The white boxes represent the locations of other figures in this paper (b) SLDEM2015, (c) SLDEM2015 slope, and (d) LOLA roughness of Mare Fecunditatis.
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Figure 3. Geological features in Mare Fecunditatis, shown with a WAC image. The green points, dark yellow triangles, blue points, red triangles, bluish green boxes, green lines, brown lines, yellow outlines, green outlines, purple outlines, and pink outlines represent identified pit craters [14], IMPs (updated from [15]), RMDSs [16], pyroclastic deposits [17,18], domes, sinuous rilles [33], wrinkle ridges [34], floor-fractured craters [35], fresh craters [20], buried craters, and swirls [19], respectively.
Figure 3. Geological features in Mare Fecunditatis, shown with a WAC image. The green points, dark yellow triangles, blue points, red triangles, bluish green boxes, green lines, brown lines, yellow outlines, green outlines, purple outlines, and pink outlines represent identified pit craters [14], IMPs (updated from [15]), RMDSs [16], pyroclastic deposits [17,18], domes, sinuous rilles [33], wrinkle ridges [34], floor-fractured craters [35], fresh craters [20], buried craters, and swirls [19], respectively.
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Figure 4. The (a) MI FeO map, (b) WAC TiO2 map, (c) IIM Al2O3 map, and (d) MI color ratio map and the age of different geological units (adapted from [12]) of Mare Fecunditatis.
Figure 4. The (a) MI FeO map, (b) WAC TiO2 map, (c) IIM Al2O3 map, and (d) MI color ratio map and the age of different geological units (adapted from [12]) of Mare Fecunditatis.
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Figure 5. Crater size–frequency distribution measurements of three Eratosthenian-aged basalt areas with IMPs (dark yellow triangles); craters with diameter > 150 m are used for dating. (a) MI color ratio map of counting area 1 (2.0° S, 43.2° E); (b) crater counting of area 1, shown with a Terrain Camera (TC, [53]) image; (c) absolute model age of area 1; (d) MI color ratio map of counting area 2 (0.4° S, 43.0° E); (e) crater counting of area 2, shown with a TC image; (f) absolute model age of area 2; (g) MI color ratio map of counting area 3 (0.1° N, 42.4° E); (h) crater counting of area 3, shown with a TC image; (i) absolute model age of area 3. The black boxes represent the locations of other figures in this paper.
Figure 5. Crater size–frequency distribution measurements of three Eratosthenian-aged basalt areas with IMPs (dark yellow triangles); craters with diameter > 150 m are used for dating. (a) MI color ratio map of counting area 1 (2.0° S, 43.2° E); (b) crater counting of area 1, shown with a Terrain Camera (TC, [53]) image; (c) absolute model age of area 1; (d) MI color ratio map of counting area 2 (0.4° S, 43.0° E); (e) crater counting of area 2, shown with a TC image; (f) absolute model age of area 2; (g) MI color ratio map of counting area 3 (0.1° N, 42.4° E); (h) crater counting of area 3, shown with a TC image; (i) absolute model age of area 3. The black boxes represent the locations of other figures in this paper.
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Figure 6. Sinuous rille, graben, and pyroclastic deposits (red dotted outline) in western Mare Fecunditatis (2.5° S, 42.0° E), shown with a WAC image.
Figure 6. Sinuous rille, graben, and pyroclastic deposits (red dotted outline) in western Mare Fecunditatis (2.5° S, 42.0° E), shown with a WAC image.
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Figure 7. Floor-fractured craters, shown with WAC images. (a) Crater Taruntius (5.5° N, 46.5° E) with dark mantle pyroclastic deposits in the center (red dotted line); (b) Crater Goclenius (10.0° S, 45.0° E).
Figure 7. Floor-fractured craters, shown with WAC images. (a) Crater Taruntius (5.5° N, 46.5° E) with dark mantle pyroclastic deposits in the center (red dotted line); (b) Crater Goclenius (10.0° S, 45.0° E).
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Figure 8. Two sites of IMP in Mare Fecunditatis, shown with Narrow Angle Camera (NAC, [13]) images. (a) IMPs next to the crater rim (2.02° S, 43.31° E); (b) IMPs on the mare (0.09° N, 42.41° E). Data source: NAC M1230441915RE and NAC M150239673LE.
Figure 8. Two sites of IMP in Mare Fecunditatis, shown with Narrow Angle Camera (NAC, [13]) images. (a) IMPs next to the crater rim (2.02° S, 43.31° E); (b) IMPs on the mare (0.09° N, 42.41° E). Data source: NAC M1230441915RE and NAC M150239673LE.
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Figure 9. The largest dome (1.5° N, 53.7° E) in Mare Fecunditatis, shown with a TC image.
Figure 9. The largest dome (1.5° N, 53.7° E) in Mare Fecunditatis, shown with a TC image.
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Figure 10. Two RMDSs (1.38° S, 52.27° E and 1.37° S, 52.29° E) in Mare Fecunditatis. (a) 3D image of these two RMDSs (five times vertical exaggeration); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1126787189LE and NAC M106533362LE.
Figure 10. Two RMDSs (1.38° S, 52.27° E and 1.37° S, 52.29° E) in Mare Fecunditatis. (a) 3D image of these two RMDSs (five times vertical exaggeration); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1126787189LE and NAC M106533362LE.
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Figure 11. Different types of craters in Mare Fecunditatis. (a) A simple crater (0.2° S, 45.5° E), shown with a TC image; (b) Messier (1.9° S, 47.7° E), Messier A (1.97° S, 46.95° E), and smooth impact melts, shown with a TC image; (c) a buried crater (3.10° N, 53.78° E), shown with a TC image; (d) crater chains on the south of Langrenus (8.8° S, 61.2° E), shown with a WAC image.
Figure 11. Different types of craters in Mare Fecunditatis. (a) A simple crater (0.2° S, 45.5° E), shown with a TC image; (b) Messier (1.9° S, 47.7° E), Messier A (1.97° S, 46.95° E), and smooth impact melts, shown with a TC image; (c) a buried crater (3.10° N, 53.78° E), shown with a TC image; (d) crater chains on the south of Langrenus (8.8° S, 61.2° E), shown with a WAC image.
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Figure 12. Pit crater (0.917° S, 48.660° E) in the central Fecunditatis. (a) 3D image (real elevation values are used); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1105602888RE and NAC M1197457075RE.
Figure 12. Pit crater (0.917° S, 48.660° E) in the central Fecunditatis. (a) 3D image (real elevation values are used); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1105602888RE and NAC M1197457075RE.
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Figure 13. Pit crater (6.752° S, 42.759° E) on the west of Fecunditatis. (a) 3D image (real elevation values are used); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1100931431LE and NAC M1120953701RE.
Figure 13. Pit crater (6.752° S, 42.759° E) on the west of Fecunditatis. (a) 3D image (real elevation values are used); (b) NAC orthoimage; (c) NAC DTM; (d) elevation of the profiles of (c). Data source: NAC M1100931431LE and NAC M1120953701RE.
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Figure 14. Swirls (14.9° S, 51.8° E) on the west of Mare Fecunditatis. (a) WAC image; (b) MI OMAT image (brighter tones indicate lower maturity); (c) MI Plagioclase abundance; (d) SVM magnetic field magnitude map.
Figure 14. Swirls (14.9° S, 51.8° E) on the west of Mare Fecunditatis. (a) WAC image; (b) MI OMAT image (brighter tones indicate lower maturity); (c) MI Plagioclase abundance; (d) SVM magnetic field magnitude map.
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Figure 15. Long-distance detection traverse in Mare Fecunditatis, shown with a WAC image. White points represent the stations in the mission, green points represent the pit craters, and lines with different colors represent segments of the traverse. Considering the error of remote sensing data, in this figure, the high-Ti basalt-traverses are defined as TiO2 > 5 wt% on the WAC TiO2 map, and high-Al basalt-traverses are defined as Al2O3 > 13 wt% on the IIM chemical map.
Figure 15. Long-distance detection traverse in Mare Fecunditatis, shown with a WAC image. White points represent the stations in the mission, green points represent the pit craters, and lines with different colors represent segments of the traverse. Considering the error of remote sensing data, in this figure, the high-Ti basalt-traverses are defined as TiO2 > 5 wt% on the WAC TiO2 map, and high-Al basalt-traverses are defined as Al2O3 > 13 wt% on the IIM chemical map.
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Table
Table 1. Travel distance, time, and speed of previous lunar rovers, the Intrepid, and the Fengfu.
Table 1. Travel distance, time, and speed of previous lunar rovers, the Intrepid, and the Fengfu.
RoverLanding TimeTravel DistanceTravel TimeAverage SpeedTop Speed
Lunokhod 1197010.54 km321 days0.97 km/lunar day0.1 km/h
Lunar Roving Vehicle (Apollo 15)197127.76 km3 h 02 min9.15 km/h13 km/h
Lunar Roving Vehicle (Apollo 16)197226.55 km3 h 26 min 7.73 km/h13 km/h
Lunar Roving Vehicle (Apollo 17)197235.89 km4 h 26 min8.10 km/h13 km/h
Lunokhod 2197339 km136 days8.46 km/lunar day2 km/h
Yutu20130.1148 km42 days80.63 m/lunar day0.2 km/h
Yutu-220181.0039 km
(10 January 2022)		1103 days26.85 m/lunar day0.2 km/h
Intrepid-1800 km4 years37.5 km/lunar day1 km/h
Fengfu (this study)-1400 km5 years22.58 km/lunar day1 km/h
Table
Table 2. Stations of the long-distance detection traverse in Mare Fecunditatis.
Table 2. Stations of the long-distance detection traverse in Mare Fecunditatis.
StationLatitudeLongitudeExploration Time (Lunar Day)Description
S1-11.31471° S52.5026° E1RMDS (High-Ti)
S1-22.16267° S52.1579° E1RMDS (Low-Ti)
S1-31.95172° S51.265° E1RMDS (Low-Ti)
S1-42.26975° S50.8258° E1RMDS and Dome
S20.917° S48.66° E5Pit crater
S32.71559° S49.0189° E1Dome
S41.94542° S47.2645° E1Impact melt
S5-11.883° S43.176° E1IMP in a crater
S5-22.16852° S43.4695° E1IMP on the mare
S63.04264° S42.348° E1Pyroclastic deposits and Sinuous rilles
S715.607° S51.9968° E1Swirl
Exploration traverse: High-Ti basalt, Low-Ti basalt, High-Al basalt, Impact blanket, and Swirl
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MDPI and ACS Style

Zhao, S.; Qian, Y.; Xiao, L.; Zhao, J.; He, Q.; Huang, J.; Wang, J.; Chen, H.; Xu, W. Lunar Mare Fecunditatis: A Science-Rich Region and a Concept Mission for Long-Distance Exploration. Remote Sens. 2022, 14, 1062. https://doi.org/10.3390/rs14051062

AMA Style

Zhao S, Qian Y, Xiao L, Zhao J, He Q, Huang J, Wang J, Chen H, Xu W. Lunar Mare Fecunditatis: A Science-Rich Region and a Concept Mission for Long-Distance Exploration. Remote Sensing. 2022; 14(5):1062. https://doi.org/10.3390/rs14051062

Chicago/Turabian Style

Zhao, Siyuan, Yuqi Qian, Long Xiao, Jiannan Zhao, Qi He, Jun Huang, Jiang Wang, Hui Chen, and Weiyang Xu. 2022. "Lunar Mare Fecunditatis: A Science-Rich Region and a Concept Mission for Long-Distance Exploration" Remote Sensing 14, no. 5: 1062. https://doi.org/10.3390/rs14051062

APA Style

Zhao, S., Qian, Y., Xiao, L., Zhao, J., He, Q., Huang, J., Wang, J., Chen, H., & Xu, W. (2022). Lunar Mare Fecunditatis: A Science-Rich Region and a Concept Mission for Long-Distance Exploration. Remote Sensing, 14(5), 1062. https://doi.org/10.3390/rs14051062

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